This invention relates generally to extrahigh-frequency (EHF) circuitry, operating at frequencies above 30 GHz (gigahertz) and, more specifically, to techniques for interfacing multiple circuit modules at those frequencies. Receivers operating in the EHF region typically employ antennas, arrays, or feed assemblies fabricated in the form of metal waveguides to provide for minimum loss and optimum performance. Metal waveguides are hollow tubes of rectangular or other cross section, through which electromagnetic energy is propagated. Propagation through waveguides is ensured by successive reflections on the conductive boundaries of the guides. Waveguides have relatively low losses, which is the principal reason they are preferred, since any losses directly degrade the overall performance of the receiver.
When long waveguide runs are required, such as from an antenna to the receiver, or if superior performance is desired, low-noise amplification is provided. Unfortunately, however, interfacing a waveguide with an active two-port amplifier device poses great difficulties. The principal one is that a waveguide has a transmission aperture that is orders of magnitude larger than the physical size of an amplifier device, such as a field-effect transistor. Moreover, input/output isolation is difficult to achieve, and provision must be made for electrical bias supplies to the amplifiers. When multiple amplifier stages are cascaded, the bias supplies have to be isolated from each other by series-connected decoupling capacitors, which increase losses in the system.
Another difficulty arises in the testing and assembly of interconnected EHF circuit modules. Typically, each module is tested separately before assembly, and test equipment for this purpose employs waveguide connections A test fixture inevitably introduces discontinuities between the fixture and the module being tested, and part of the testing procedure involves tuning the module to achieved a desired performance. Waveguides are relatively inconvenient to tune, since tuning involves adjustments of some kind to the physical waveguide structure. A more important problem is that, when the separately tested modules are subsequently assembled together as a system, the discontinuities that were present during testing are replaced by different discontinuities, and further tuning is usually required to obtain the desired performance.
It will be appreciated from the foregoing that there is a need for an EHF circuit module that eliminates or minimizes these problems. Ideally, the circuit module should employ an external waveguide interface, to provide low losses and ease of coupling with other modules and with test equipment, but should also provide for convenient coupling to active two-port devices, such as amplifiers. It is also very important that the circuit module be easily tunable when assembled in its final configuration. As will now be summarized, the present invention is directed to these ends.